This is a Competitive Revision Application in response to the Recovery Act Funds Notice Number NOT-OD-09- 058. The parent project over goal is to explore the biology, biochemistry and pathophysiology of N- acetylglutamate synthase (NAGS) is an enzyme that produces the cognate cofactor N-acetylglutamate (NAG), an essential allosteric activator in ureagenesis. This revision application adds two new aims to the parent project. 1. To develop a knockout mouse for NAGS deficiency, characterize its phenotype, and explore its use as a conditional hyperammonemia model. 2. To "isolate" and study the in vivo regulation of ureagenesis specifically at the level of NAGS/CPSI by comparing nitrogen metabolism in N-carbamylglutamate (NCG) treated koNAGS mouse vs. wild type littermates. In addition to these new aims to be completed in two years, this project will enhance in the long term two of the existing aims that study the arginine effects on NAGS function and the effect of naturally-occurring mutations in patients with NAGS deficiency. In this revised project, we will develop, study and make available to the research community a novel "titratable mouse model of hyperammonemia. This knockout NAGS mouse will be rescued with NCG and will develop hyperammonemia upon withdrawal of this cofactor analog. We will determine the in vivo differences between nitrogen balance and metabolism in the NAGS "regulation-deprived" koNAGS mice rescued with NCG vs. the naturally-regulated wild type littermates on and off NCG. These studies will use both gene expression and protein profiles methods and will allow for the first time to "isolate" in vivo the regulatory effects of NAGS on ureagenesis. This project will enhance the pace and quality of the parent grant by providing a new tool for in vivo investigations for our group and other investigators studying hyperammonemia. In addition, The contribution of this project to the economy is leveraged by making the koNGAS mouse model available to othr investigators across the country, enhancing their research and promoting new job creation. PUBLIC HEALTH RELEVANCE: This project is dedicated to the investigation of an important gene and protein (NAGS) that determined how much nitrogen we eliminate from our bodies. It is important to know this since one of the main problem in liver disease is the inability to eliminate toxic nitrogen (ammonia) which can poison the brain. We will study an engineered mouse that does not posses NAGS to allow us to better understand this system and how it is regulated. The results from this project could allow the development of new treatments for elevated ammonia levels to protect the brain from its toxic effects.